A solid-state laser device excited by a semiconductor laser for causing laser oscillation by exciting a solid laser crystal by a laser beam from a semiconductor laser light source, has characteristics of having a small size, a light weight, a long life, and a high electrical/optical conversion efficiency, a stable operation and the like and is used in various industrial fields.
FIG. 18 is a diagram showing the construction of a conventional internal resonator type solid-state laser device 101 excited by a semiconductor laser. A pump beam 119 having a wavelength of about 808 nm and emitted form a semiconductor laser light source 110 passes through a wavelength locking device 112 and is incident on a solid laser crystal 115 via a coupling lens system 113 after being collimated substantially to a parallel beam by a collimator lens 111. A reflecting mirror 116 for a fundamental wave is formed on one end surface of the solid laser crystal 115.
A fundamental wave 120 having a wavelength of about 1064 nm is output from the solid laser crystal 115 excited by the pump beam. The fundamental wave 120 oscillates in an internal optical resonator formed by a reflection coating 123 formed on an output mirror 117 and the reflecting mirror 116. This fundamental wave 120 is incident on a wavelength conversion element 118 made of a nonlinear optical medium, whereby second harmonic components (harmonics 121, 122) of the fundamental wave 120 are generated. The generated harmonics 121, 122 are emitted to the outside via the output mirror 117.
If such a solid-state laser device is used, a high-output green beam can be obtained. As a specific construction example, a solid laser crystal made of, e.g. Nd:YVO4 is excited using, for example, a semiconductor laser light source to induce laser oscillation of the solid laser crystal between a reflecting mirror and an output mirror. By this laser oscillation, a fundamental wave having a wavelength of 1064 nm is obtained. This fundamental wave is incident on a wavelength conversion element, whereby a second harmonic having a wavelength of 532 nm is obtained. Since a high-output green beam can be obtained by employing such a construction, this solid-state laser device can be applied to a display or the like using a laser, wherefore developments on this are being actively made.
FIG. 19 is a diagram showing a schematic construction of a conventional image display device. An image display device 201 is provided with a red light source 202, a green light source 203 and a blue light source 204. The red and blue light sources are constructed by semiconductor lasers. The green light source is constructed by an internal resonator type solid-state laser device excited by a semiconductor laser. Laser beams output from the respective light sources pass through a uniformizing optical system 205 to be incident on a polarization beam splitter 207 after being reflected by a dichroic mirror 210. Thereafter, the polarization beam splitter 207 reflects the laser beams toward an image conversion device. A reflective liquid crystal panel 206 is used in the image conversion device. The laser beams incident on the reflective liquid crystal panel 206 are reflected in accordance with a video signal and pass through an exit lens 208 to be output as a video image.
The outputs of the respective light sources are controlled by a control circuit 209. The image display device 201 also includes a battery 211 and can be battery driven. By using the lasers as the light sources, increased color reproducibility, instantaneous start-up, miniaturization of the device can be realized as compared with a conventional device using lamps.
Several technologies have also been proposed for output stabilization of a semiconductor laser excited solid-state laser device. In the internal resonator type semiconductor laser excited solid-state laser device shown in FIG. 18, an output may become unstable due to the re-incidence of harmonics on the wavelength conversion element.
In FIG. 18, there are two harmonics; the harmonic 121 generated by the fundamental wave propagating from left to right and the harmonic 122 generated by the fundamental wave propagating from right to left. In order to effectively utilize generated harmonics, the reflecting mirror 116 is generally a coat for highly reflecting harmonics. However, in this case, the harmonic 122 is incident again on the wavelength conversion element 118 after being reflected by the reflecting mirror 116. At this time, the harmonic is partly reversely converted into a fundamental wave. The phase of the reversely converted fundamental wave slightly deviates from that of the original fundamental wave in some cases, whereby the reversely converted fundamental wave and the original fundamental wave interfere to vary a fundamental wave output. As a result, a harmonic output becomes unstable as shown in FIG. 20.
FIG. 20 is a graph showing destabilization of a harmonic output caused by reverse conversion of a harmonic into a fundamental wave in an optical resonator. As shown in FIG. 20, as the amount of a pump beam increases, the harmonic output increases and, after temporarily decreasing, increases again. Since the harmonic is reversely converted into the fundamental wave in the optical resonator in this way, the reversely converted fundamental wave and the original fundamental wave interfere to make the harmonic output unstable.
A construction for absorbing one harmonic is proposed in patent literature 1 in connection with the above problem. Further, a construction as shown in FIG. 21 is used in patent literature 2. FIG. 21 is a diagram showing the construction of a conventional solid-state laser device realizing harmonic output stability.
A pump beam 302 emitted from a semiconductor laser light source 301 is obliquely incident on a solid laser crystal 303. A fundamental wave 306 is generated in an optical resonator formed by two reflecting minors 304, 307 and a part of the fundamental wave 306 is converted into a harmonic in a wavelength conversion element 305. A harmonic 308 generated from the fundamental wave propagating from left to right in FIG. 21 is output after passing through the reflecting mirror 307. The reflecting minor 307 exhibits a low reflection for harmonic.
On the other hand, a harmonic 309 generated from the fundamental wave propagating from right to left in FIG. 21 is output after passing through the reflecting minor 304. The reflecting mirror 304 also exhibits a low reflection for harmonic. By the above construction, the two harmonics are extracted in different directions to prevent a bad influence on the fundamental waves.
However, since one harmonic is absorbed in the example of patent literature 1, the harmonic output becomes about half, wherefore efficiency is poor and it is difficult to obtain a high output. Further, in the example of patent literature 2, the harmonics are extracted in right opposite directions. Thus, the direction of either one of the beams needs to be changed if it is tried to use both of the two output harmonics, which leads to an increase in optical parts and a cost increase.
[Patent Literature 1]
    Publication of Japanese Patent No. 3222288[Patent Literature 2]    Japanese Unexamined Patent Publication No. 2006-186071